1. Technical Field
This invention relates to a method of bonding layers together and in particular relates to a method of bonding semiconductor substrates and device layers together as well as the adhesive bonding layer itself.
2. Background Information
The ability to bond semiconductor materials together has become increasingly important for improving device applications, such as those of high-brightness light emitting diodes (LEDs). Until recently, non-semiconductor materials such as polymer, ceramics and metals have been used as bonding agent (also referred to as a bonding layer) between the two structures to be bonded. Similarly, semiconductor materials provided through epitaxial growth have been used in the bonding process. Each of these materials has disadvantages, however.
Use of epitaxial growth for bonding is especially problematic in certain respects. For example, if single crystal grown material is used and this material has a different lattice constant from the single crystal substrate on which it is grown, strain builds up in the grown material. If this strain is high and the thickness of the grown material exceeds a critical thickness, misfit and threading dislocations that relieve the strain will form in the material. Such defects damage the electrical and optical properties of the material and impede the performance of devices formed with the structure. Selection and growth of particular material that is lattice matched to substrates thus presents one challenge in using epitaxially grown material.
Conventional bonding processes using semiconductor materials have basically consisted of putting two semiconductor wafers together and applying force at certain temperatures to the wafers to make them fuse together. The most extensively used wafer bonding approach used currently is direct wafer bonding. In this technique, the bonding is achieved without any bonding agent between the two semiconductors.
Direct bonding processes employ two different techniques. The first approach consists of contacting one wafer, such as GaAs, and another wafer (for example Si or GaAs) in solution. The two wafers are bonded by natural intermolecular surface forces (Van der Waals bonding). Unfortunately, this type of bonding usually results in very poor current conduction through the bonding interface, thereby limiting the usefulness of the technique. In addition, the bonds formed using this technique are generally very weak unless they are subjected to a high temperature heat treatment (anneal) to convert the Van der Waals bonding into chemical bonding, that is, temperatures well in excess of about 600° C. Unfortunately, large thermal stresses produced during such high temperature annealing can cause problems, in part at least due to differences in the thermal expansion coefficients between the different materials. Such a problem is also present when bonding agents are present between the wafers.
In another approach, two single crystal surfaces of different materials are brought together in contact in an N2 or H2 environment with applied pressure and heat treatment at a temperature higher than that of the above technique. Given a sufficient amount of annealing, these two wafers fuse together. This bonding process involves surface-energy-induced migration and growth, in which interdiffusion of the constituent molecules or atoms of the two materials occurs at the surface. Examples of structures using direct wafer bonding include InP-based semiconductor laser diodes on Si or GaAs substrate which show a uniform bonding interface.
In this approach, although the temperature that must be used varies with the materials used, high temperatures in excess of 600° C. for GaAs and InP, 800° C. for GaP, and 1000° C. for GaN-based materials are used during the heat treatment to ensure that the bonding is strong enough to withstand the further processing. Besides the thermal stresses generated, in many situations involving compound semiconductors, such high temperatures cause the semiconductor materials to structurally break down due to the migration of atoms between layers. This consequently lowers the electrical performance of devices which rely on relatively sharply defined interfaces of the crystalline structure and dopants in the structure.
In addition, as mentioned above, conventional wafer fusion requires strict physical alignment between the structures prior to bonding so that bonding interface can be freely conducting, as shown in the examples of FIGS. 1a and 1b. As illustrated in the graphs of FIGS. 1a and 1b, even a slight misalignment of 6° or less between n-GaP and n-InGaP layers causes a relatively large amount of deviation from pure ohmic characteristics. Thus, alignment is crucial to direct wafer bonding technology since misalignment affects the amount of current flowing through the device. If enough misalignment exists, the number of die that may be used to form the final device decreases, the yield decreases, and the cost consequently increases. Also, extremely flat surfaces are required for the bond to be electrically conductive.
An alternative method to direct wafer bonding, as previously noted, is using a foreign material as a bonding agent, such as Au—Ge metal, spin-on-glass (SOG) or some organic adhesion layer such as an epoxy, photoresist or polyimide. In general, this bonding technique is performed by first depositing the bonding agent on both wafers to be bonded. The bonding agent can be in liquid form and solidified while bonding or be deposited in a solid form. The surfaces are brought in contact and pressed together under a particular pressure at a set temperature. The bonding agents fuse together and hold wafers to achieve heterogeneous integration.
A bonding process that uses such a bonding agent can be performed at lower temperatures than direct wafer bonding. In addition, the bonding is generally chemically and thermally stable at the lower temperatures used. Unfortunately, the use of foreign bonding materials has a number of detrimental consequences in many device applications. Most notably, this technique is not amenable to the subsequent chemical and thermal processes that are employed in semiconductor device manufacturing, such as metal alloying or chemical vapor deposition (such as that used in post-bond growth). High temperatures or temperature cycling used in these later processes cause many of the materials used for bonding to break down, or at least weaken substantially. If an organic adhesion layer is used, additionally such a layer in general changes chemistry upon drying or heating. Furthermore, chemicals, such as acetone, commonly used in further processing dissolve many of such organic adhesive materials. Other materials and specific problems include SOG layers, which have reliability problems or metallic bonding layers, which block light from being transmitted through the bonding layer. The latter, of course, limits the use of the metallic bonding layer in optoelectronic integrated circuit applications. Metal bonds may also undergo extensive alloying in high temperature processing. In addition, most of the materials used in this bonding technique make the bonding electrically isolating; that is, electrical current cannot be conducted across the bond.
Despite each of these techniques having its own problems, conventional direct wafer bonding or wafer bonding with a bonding agent are used in many applications. One of these applications includes LED production, as mentioned above. In LED production, using the direct wafer bonding process, the amount of light from the device may be increased by a factor of 2-3 by wafer bonding the LED to a substrate that is transparent to the wavelength of light emitted by the LED. In one example of this technique, an LED structure that includes an active region of AlGaInP on a GaAs substrate followed by a 50-μm GaP layer is grown using vapor phase epitaxy (VPE). Next, the substrate is etched off, leaving just the active region and the GaP support layer. The GaP layer is used both for support after the GaAs substrate is removed as well as for current spreading. A high density of dislocations form in the GaP current-spreading layer due to the lattice mismatch between GaP and the AlGaInP, but since it is not part of the light emitting region it does not affect the performance. Finally, a GaP substrate, which is transparent to the light emitted by the LED structure, is bonded to the active region at a temperature of at least 800° C. The GaP substrate is transparent as it is an indirect bandgap material whose bandgap is about 2.2 eV, which is larger than the energy of photons emitted by the active region. A similar process having similar results can be achieved in the formation of vertical cavity surface-emitting lasers (VCSELs) used for long-range fiber optic communications.
While the results obtained are desirable, the amount of time spent processing the device, and thus the overall cost of the device, increases dramatically as a result of the exactitude necessary during processing. The growth, wafer alignment, and structural thinning as well as handling of the thinned structure all require a large amount of time and care to maintain a suitable device yield.
It would thus be advantageous to provide a bonding arrangement and method that is relatively simple to grow, in which the bonding layer can be grown on any surface, the various substructures within the overall bonded structure are independent of orientation relative to each other, and in which sensitive preparations are not necessary. Further, it would be advantageous to provide an arrangement and method that is optically transparent for a large range of useful wavelengths, electrically conductive, as well as being performed at a relatively low temperature compared with conventional bonding processes. All of these lead to a product having decreased cost.